879 
demonstrated that it is erroneous to assert that commercial alumi- 
nium is covered with a coating of oxide. It was shown before that 
when the bottom of a vessel with an Al, (SO), solution is covered 
with a layer of mercury, and when through the solution an alumi- 
nium rod is immersed in the mercury layer, the aluminium rod 
immediately assumes the potential of the mercury, from which follows 
that the aluminium rod was not covered with an insulating layer 
of Al,O,, but was in direct contact with the mercury ‘). 
Now that this fact has been established, and the initial condition 
is uncovered metal, it is clear that it must be explained why on 
anodic polarisation the potential of the metal becomes so strongly 
positive already with very small current densities that the tension 
of separation of the oxygen is reached. lt is seen that here the 
same question presents itself as in the case of anodic polarisation of 
other inert metals. It was pointed out before that in the first place 
the most essential, the primary phenomenon, should be explained 
viz. the change of the potential in noble direction; the oxygen, 
separation and the subsequent oxide formation are secondary phenomena. 
The strong ennobling of the potential of aluminium on anodie 
polarisation must be explained by this, that while the withdrawal 
of electrons from the metal which is represented by 
36, 
is immediately followed by aluminium-ions going into sulution 
Al. 
{ 
Al, 
because this heterogeneous equilibrium is instantaneously established, 
the homogeneous reaction 
Al, > Als +3 6. 
proceeds with very small velocity, so that the metal becomes poorer 
in ions and electrons. In consequence of this the potential of the 
metal becomes less negative or more positive, aS appears from the 
equation : 
0,058 Ly 
== log : — 2,8 
B (ML) 
because in this case Ly, becomes smaller. 
This phenomenon is, therefore, primary, and if the metal is inert, 
') Swits, ATEN. I.c. 
Proceedings Royal Acad. Amsterdam Vol. XXII. 
